MUSEUM OF COMPARATIVE ZOOLOGY. 105 



3. — General Features of Intrusive and Extrusive Sheets in 



Connecticut. 



GROUP I. WESTERN RIDGES. 



Locality 1. Section numbers, 85-89. Local name, East Rock. Percival's Report, 

 pp. 395-398. Percival's notation, W. S. 1. (1). 



General Account. — The southwestern face of this fine mass is well 

 exposed in a strong palisaded cliff on the border of New Haven, below 

 which the underlying sandstones can be seen at several points. The over- 

 lying sandstone close to its contact with the trap was found on the 

 northeastern slope, in the woods, about a third way down from the sum- 

 mit. Tliis rock is regarded as a part of the West Rock sheet, from 

 which it is thought to have been separated by a fault ; similar faults 

 of smaller throw are supposed to account for the notches in the south- 

 eastern extension of East Rock itself. 



Sections cut from specimens taken from the upper contact and from 

 four feet below it cannot be distinguished from sections similarly selected 

 from the base of the sheet. 



The trap is wanting in vesicles of expansion throughout its mass, and 

 is hclocrystalline except at contact with other rocks. Extremely close- 

 grained and glassy at the tipper contact, where it shows microscopic 

 flowage parallel to surface of junction with the overlying sandstone. 

 Sandstone directly above does not contain fragments of trap ; hand 

 specimens appear much more dense than from beds distant from the 

 trap sheet. 



Locality 2. West Rock. Percival's Report, pp. 394-396. Percival's notation, 



W. S. I. (4). 



The general features of this ridge are like those of East Rock ; but 

 no exposure of the upper contact has been found on its back. As far as 

 seen, it is of dense texture, even in the uppermost parts exposed. The 

 southern end of the ridge, where the underlying sandstone is quarried 

 and exposed in contact with the trap, may be reached by the West 

 Haven horse cars from ]^ew Haven ; the remainder of the ridge is 

 wooded and less easily examined. 



Locality 3. Section numbers, 194-199. Gaylord's Mountain, Roaring Brook. Perci- 

 val's Report, pp. 402-404. Percival's notation, W. S. II. 



Gaylord's Mountain is a slightly dislocated continuation of the West 

 Rock range ; on its back, Roaring Brook has cut a picturesque ravine, 



